1. Field of the Invention
The present invention relates generally to electronic devices that utilize elements that exhibit bistable electrical behavior. More particularly, the present invention is directed to organic semiconductor devices including electrically programmable nonvolatile memory devices and switches.
2. Description of Related Art
The publications and other reference materials referred to herein to describe the background of the invention and to provide additional details regarding its practice are hereby incorporated by reference. For convenience, the reference materials are numerically referenced and identified in the appended bibliography.
Many electronic memory and switching devices typically employ some type of bistable element that can be converted between a high impedance state (off-state) and a low impedance state (on-state) by applying an electrical voltage or other type of writing input to the device. This threshold switching and memory phenomena have been demonstrated in both organic and inorganic thin-film semiconductor materials. For example, this phenomenon has been observed in thin films of amorphous chalcogenide semiconductor (1), amorphous silicon (2), organic material (3) and ZnSe—Ge heterostructures (4).
The above materials have been proposed as potential candidates for nonvolatile memories. The mechanism of electrical bistability has been attributed to processes such as field and impact ionization of traps, whereas in chalcogenide semiconductors they involve amorphous to crystalline phase changes. Analogous memory effects in the leakage current of ferroelectric BaTiO3 or (Pb1-yLay)(Zr1-x)O3-based heterostructures have also been reported and discussed in terms of band bending due to spontaneous polarization switching. Electrical switching and memory phenomena have also been observed in organic charge transfer complexes such as Cu-TCNQ[5,6].
A number of organic functional materials have attracted more and more attention in recent years due to their potential use in field-effect transistors (7), lasers (8), memories (9,10) and light emitting diodes and triodes (11,15). Electroluminescent polymers are one of the organic functional materials that have been investigated for use in display applications. In addition to display applications, electroluminescent polymers have been doped with high dipole moment molecules in order to obtain a memory effect (12). This memory effect is observed when dipole groups attached to side chain of the polymer rotate due to application of a threshold bias voltage. Unfortunately, rotation of the dipole groups takes a relatively long time. Also, doping of the polymer reduces the electroluminescence of the doped polymer.
Electronic addressing or logic devices are presently made from inorganic materials, such as crystalline silicon. Although these inorganic devices have been technically and commercially successful, they have a number of drawbacks including complex architecture and high fabrication costs. In the case of volatile semiconductor memory devices, the circuitry must constantly be supplied with a current in order to maintain the stored information. This results in heating and high power consumption. Non-volatile semiconductor devices avoid this problem. However, they have the disadvantage of reduced data storage capability as a result of higher complexity in the circuit design, and hence higher cost.
A number of different architectures have been implemented for memory chips based on semiconductor material. These structures reflect a tendency to specialization with regard to different tasks. Matrix addressing of memory location in a plane is a simple and effective way of achieving a large number of accessible memory locations while utilizing a reasonable number of lines for electrical addressing. In a square grid with n lines in each direction the number of memory locations is n2. This is the basic principle, which at present is implemented in a number of solid-state semiconductor memories. In these types of systems, each memory location must have a dedicated electronic circuit that communicates to the outside. Such communication is accomplished via the grid intersection point as well as a volatile or non-volatile memory element which typically is a charge storage unit. Organic memory in this type of matrix format has been demonstrated before by using an organic charge transfer complex. However such organic memories require transistor switches to address each memory element leading to a very complex device structure.
Organic Electrical Bistable Devices (OBD's) have been proposed in the past where a metal layer is sandwiched between two organic layers. This sandwich structure is used as an active medium that is interposed between two electrodes. Controllable memory performance has been obtained using this type of configuration. A positive voltage pulse is used for writing, while a reversed bias is used for erasing. The shortcoming of this kind of memory device is that erasure must be performed by applying a reversed bias. In an x-y electrical-addressable memory array application, a diode must be series connected with each memory cell to prevent the so-called “sneak current”. In this type of application, it is difficult to apply a reversed bias for erasing. In addition, the middle metal layer makes it technically difficult to pattern the metal layer for each memory cell when the cells are very small.
The diffusion or drift of Cu-ions into semiconductor materials, like silicon, is a well-known and troublesome phenomenon that has an adverse effect on semiconductor devices (16). Generally a diffusion barrier layer is added to prevent Cu metallization (17). Electrical-addressable nonvolatile memory devices have attracted considerable attention in recent years due to their application in information technology. Silicon based floating-gate memory (18), with a response time in the sub-millisecond, has played an important role in the modern electronic devices, such as digital cameras. However, there is always a strong demand for electronic nonvolatile memory devices that are less expensive and better. Organic electrical bistable devices are promising in this regard.
Organic electrical bistable devices with an organic/metal-nanocluster/organic tri-layer structure sandwiched between two electrodes have been made (19). These sandwich structures show nonvolatile memory behavior. Many other methods have also been reported for nonvolatile memory, such as phase change memory (20), programmable metallization cell (21), nano-crystal memory (22), organic memory based on scanning probe microscope (23), and organic memory in charge-transfer complex system (6), polystyrene films (24), and molecular devices (25).
In view of the above, there is a continuing need to provide new and improved electrically bistable structures which may be used in electronic devices, such as memory devices and switches.